A novel member of the steroid/thyroid hormone superfamily of nuclear receptors has recently been identified whose expression is limited to the germ cells. This factor, named germ cell nuclear factor (GCNF) binds to specific DNA sequences which resemble the binding sites of other nuclear receptor transcription factors. Because GCNF is specific to the haploid gametes, its function has both clinical and basic science significance. Clinically, it may direct pathways for novel, highly specific contraceptives, or for underlying causes of infertility. GCNF may also contribute to regulation of meiosis because of its limited range of expression. To directly address these questions, I propose to mutate the GCNF gene in mice. The genomic GCNF clone will be obtained, and a restriction map produced. A targeting vector including 5-8kb of GCNF sequence, a positive selection gene, replacing GCNF exonic sequences, and a negative selection gene, placed downstream of GCNF homologous sequences, will be constructed to insure a double crossover homologous recombination event in transfected embryonic stem cells. After Southern blot screening, cello with integrated vector DNA will be micro-injected into growing mouse blastocysts. The resulting chimeric mice will be screened for integration into the germ line and bred to produce homozygous mutant mice. Histological and in situ hybridization analysis, will be performed to determine the effect of GCNF on germ cell development and function.